Method for increasing the consistency of pulp using a single roll displacment wash press

ABSTRACT

A single roll wash press comprising a housing and a drum rotatably mounted within the housing. The drum includes a perforated outer wall and a longitudinally extending centerline axis and is rotatable about the axis. The press further comprises a means for receiving a flow of pulp having an initial consistency ranging from about 2% o.d. to about 6% o.d. such that the pulp flows in the direction of drum rotation, and a means for defining a divergent formation zone and for forming a mat of the pulp on the drum within the divergent formation zone. The means for defining and forming comprises a first arcuate portion of the housing with the formation zone being divergent between upstream and downstream ends thereof. The upstream end of the divergent formation zone communicates with the means for receiving the flow of the pulp. The press further includes a means for washing the pulp mat within a displacement zone disposed downstream of the extraction zone. The press may further include a means for increasing the consistency of the pulp within an extraction zone disposed circumferentially between the formation and displacement zones.

CROSS-REFERENCES

This is a divisional of application Ser. No. 08/689,184, filed Aug. 5,1996, and now U.S. Pat. No. 5,722,264.

The subject application claims the priority benefits of U.S. Provisionalpatent application having Ser. No. 60/001,946 filed on Aug. 7, 1995,entitled: "Single Roll Displacement Wash Press".

BACKGROUND OF THE INVENTION

1.0. Field of the Invention

The present invention relates generally to the washing oflignocellulosic materials, such as wood pulp, for use in the pulp andpaper industry, and more particularly to a single roll displacement washpress.

2.0. Related Art

Washing of lignocellulosic materials such as wood pulp is well known inthe art. The washing of pulp is typically accomplished by a combinationof two mechanisms: dilution-extraction; and displacement. Displacementwashing typically is more efficient than dilution-extraction washing,since less wash medium is required to accomplish the same degree of pulpcleanliness. Although displacement washing has been widely used in thepulp and paper industry, it is subject to the following limitations.Effective displacement washing requires that the pulp fiber mat that isto be washed is of uniform density and free from discontinuities. If thepulp mat does not have these characteristics, the washing medium may bechanneled through the pulp mat, thereby defeating the purpose ofdisplacing undesirable contaminated liquor with relatively clean washmedium. Additionally, the fiber mat to be washed must be resilient so asto maintain the qualities of uniform density and lack of discontinuityas the wash medium is applied.

Known pressure rotary drum washers and presses typically form a fibermat on a perforated drum surface by feeding pulp having a consistencyranging from about 3% o.d. to about 6% o.d. to a formation zone. In allpressure-type rotary drum washers and presses having a wash pond, whichare known to the inventor, the formation zone converges in the directionof flow of the pulp through the formation zone. As the pulp stockprogresses through the convergent formation zone, the pulp mat is formedand dewatered through perforations in the drum to a higher consistency.A relatively high differential pressure between the formation zone andthe interior of the drum is required for the rapid dewatering of thepulp mat to occur. The resulting pulp mat is then discharged to a washzone containing the wash medium. The wash zone is necessarily at a lowerpressure than the formation zone so that the pulp may exit the formationzone and to prevent the wash medium from entering the formation zone.The rapid pressure drop that the pulp mat experiences causes disruptionof the mat. Some of the pulp may be so violently disrupted that itliberates from the mat and mixes freely with the wash medium. Theresulting discontinuities in the pulp mat and the mixing of a portion ofthe pulp mat with the wash medium prevents full exploitation of thedisplacement process. Additionally, in known pond-type pressure washersand presses, the wash liquid must be introduced into the displacementzone at a relatively high velocity in order to achieve effectivedistribution of the wash liquid. The wash liquid inlet is typicallylocated immediately following the formation zone where the highturbulence of the wash liquid further disrupts the pulp mat anddecreases the efficiency of the displacement process.

Known wash presses typically employ two drum surfaces which rotate withone another to form a pressing nip. A pulp mat is formed and washed, asdescribed previously, on the surfaces of one or both of the drums. Inall wash devices which achieve a high final discharge consistency of thepulp, the wash liquor is applied while the pulp fiber mat is at anintermediate consistency, typically ranging from about 9-18% o.d., inorder to achieve practical wash liquor supply pressures. Consequently,due to the relatively high volume of liquid in the medium consistencypulp and the requirement to maintain a system flow balance between theliquid supplied to and extracted from the wash device, the amount ofwash liquor applied is typically insufficient to fully displace thecontaminated liquor contained in the mat formed from the mediumconsistency pulp. This problem is exacerbated in the case of twin rollpresses, where the partially washed pulp mat is fed into a nip formed bytwo rolls and pressed to a high consistency. The rate of convergence atthe nip is so great that extremely rapid dewatering must take placerequiring extremely high pressures. The pulp mat typically cannotwithstand the high pressures and micro-disruptions in the pulp fibermatrix form which permits the wash liquor to flow through the pulp matin an undesirable, random manner. Uniform dewatering cannot take placein this environment and the boundary between the wash liquor and thecontaminated liquor in the pulp is not maintained, thereby eliminatingthe possibility of continuing displacement washing during the extractiondewatering process at the nip which reduces overall washing efficiency.

Additionally, with known pressure washers the pulp mat passes through atleast one converging area which may be created by a dewatering baffle,press roll, or similar device prior to being discharged from the washer.In the case of washers which include a dewatering press roll or similardevice which is substantially non-retractable, it is virtuallyimpossible to purge any pulp plug which may be formed in the machine. Inthe case of washers having a substantially retractable baffle, thebaffle discharge is typically substantially displaced from the finaldischarge area of the washer such that any plug which may be liberatedfrom the baffle is required to travel upward in the washer against theaction of gravity while passing other washer components that may tend totrap the plug before it may be purged from the washer.

The foregoing illustrates limitations known to exist in present woodpulp washers. Thus, it is apparent that it would be advantageous toprovide an alternative directed to overcoming one or more of thelimitations set forth above. Accordingly, a suitable alternative isprovided including features more fully disclosed hereinafter.

SUMMARY OF THE INVENTION

In one aspect of the present invention, this is accomplished byproviding a single roll displacement wash press, comprising:

a housing;

a drum rotatably mounted within housing, the drum including alongitudinally extending centerline axis, a generally cylindrical outerwall having an outer surface and an inner surface, wherein the outerwall is perforated and the drum is rotatable about the centerline axis;

a means for receiving a flow of pulp having a consistency ranging fromabout 2% o.d. to about 6% o.d. such that the pulp flows in the directionof rotation of the drum;

a means for defining a divergent formation zone and for forming a mat ofthe pulp on the drum within the divergent formation zone, the means fordefining and forming comprising a first arcuate portion of the housing,wherein the divergent formation zone is divergent between an upstreamend thereof and a downstream end thereof, the upstream end of thedivergent formation zone communicating with the means for receiving theflow of the pulp;

a means for washing the pulp mat within a displacement zone, thedisplacement zone being disposed downstream of the divergent formationzone.

In a second aspect of the present invention, this is accomplished by amethod for increasing a consistency of pulp using a single rolldisplacement wash press having a housing and a drum rotatably mountedwithin the housing, with the press further including in circumferentialsuccession, a formation zone, an extraction zone, and a displacementzone, and with the method comprising the steps of:

supplying a pulp having a consistency ranging from about 2% o.d. toabout 6% o.d. to the formation zone of the single roll displacement washpress;

forming a mat of the pulp on a generally cylindrical outer surface ofthe drum within the formation zone of the press;

supplying a gaseous medium to the extraction zone so as to pressurizethe extraction zone and gently force contaminated liquor out of the pulpmat so as to increase the consistency of the pulp within the extractionzone;

washing the pulp mat within the displacement zone of the press;

removing the pulp mat from the drum.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other aspects of the present invention will becomemore apparent from the subsequent Detailed Description when consideredin conjunction with the accompanying drawings, wherein:

FIG. 1 illustrates a single roll displacement wash press according to afirst embodiment of the present invention;

FIG. 2 illustrates a single roll displacement wash press according to asecond embodiment of the present invention.

DETAILED DESCRIPTION

Referring now to the drawings, FIG. 1 illustrates a single rolldisplacement wash press 10, according to a first embodiment of thepresent invention. The wash press 10 is effective for washinglignocellulosic materials, such as wood pulp. As known in the art, woodpulp is obtained from the digestion of wood chips, from repulping ofrecycled paper, or from other sources and is commonly processed in pulpand paper mills in slurry form in water. As used herein, the term"consistency" is used to express the measured ratio of dry pulp fibersto water, or more specifically, the weight of dry pulp fibers in a givenweight of pulp slurry or "pulp stock" as a percentage. Variousdefinitions are used, such as air-dry consistency (a.d. %) or oven-dryconsistency (o.d. %), or moisture-free consistency (m.f. %). Thelaboratory techniques for measuring these values can be found inreferences well known in the art, such as the TAPPI Standards Manual.Terms widely used to describe ranges of stock consistency useful in pulpand paper plants follow:

Low Consistency--below about 4-6% o.d.

Medium Consistency--about 9-18% o.d.

High Consistency--above about 18-20% o.d., but more commonly above about25% o.d.

The single roll displacement wash press 10 includes a housing 12 and aperforated drum 14 which is rotatably mounted with housing 12. The drum14 includes a generally cylindrical outer wall 16 and a generallycylindrical inner wall 18 which is radially spaced from and inward ofthe outer wall 16. Drum 14 further includes a plurality ofcircumferentially spaced struts 20 which define, or form a plurality ofcircumferentially spaced filtration chambers 22 between the outer wall16 and the inner wall 18 of drum 14. Filtration chambers 22 extendsubstantially throughout the longitudinal length of drum 14 and areeffective for draining contaminated liquor from drum 14 as subsequentlydescribed in further detail. Drum 14 further includes a plurality ofgenerally radially extending filtration holes 24 which extend throughthe outer wall 16 of drum 14. Each of the holes 24 extends between agenerally cylindrical outer surface 26 of outer wall 16 and a generallycylindrical inner surface 28 of outer wall 16 and communicates with oneof the filtration chambers 22. Drum 14 further includes a longitudinallyextending centerline axis 30 and is rotatable about axis 30. In theillustrated embodiment shown in FIG. 1, drum 14 rotates in a clockwisedirection about axis 30.

A flow of a pulp 32 having a consistency ranging from about 2% o.d. toabout 6% o.d., is supplied to a pulp inlet 34 of the housing 12 of press10. The low consistency pulp 32 includes a relatively high volume ofcontaminants due to upstream processing within the associated pulpplant. The removal of these contaminants is accomplished by the washpress 10 of the present invention as subsequently described in furtherdetail. The low consistency pulp 32 supplied to inlet 34 then flows to apulp slurry inlet box 36 which extends substantially throughout thelongitudinal length of wash press 10. A bar-type slice valve 38 isrotatably mounted within the pulp slurry inlet box 36. When valve 38 isin the open position illustrated in FIG. 1, the pulp slurry inlet box 36communicates with a formation zone 40 of press 10, so that the lowconsistency pulp 32 discharges from box 36 through valve 38 (the portionnot shaded in FIG. 1) and into the formation zone 40. The pulp slurry,or low consistency pulp 32 then immediately follows the rotation of thedrum 14 which is in a clockwise direction in the illustrative embodimentshown in FIG. 1. The flow of the pulp 32 within housing 12 in thedirection of the rotation of drum 14 is facilitated by a flow deflector39 which is attached to a member 41 of inlet box 36 which extendssubstantially throughout the longitudinal length of wash press 10. Dueto the rotation of drum 14, a pulp mat 42 is formed on the outer surface26 of the outer wall 16 of drum 14. The pulp mat 42 is then exposed, insuccession, to an extraction zone 44, a displacement zone 46, acompaction zone 48, and a face cleaning and take-off zone 50 of press10, within housing 12, due to the rotation of drum 14. Thecircumferential boundaries of each of the zones 40, 44, 46, 48, and 50are schematically indicated generally by a pair of outwardly extendingradial lines emanating from axis 30 of drum 14 as shown in FIG. 1.

The outer boundary of the formation zone 40 is formed, or defined by afirst arcuate portion 52 of housing 12 with the pulp mat 42 being formedon the outer surface 26 of the outer wall 16 of drum 14. The pulp mat 42extends radially between the outer surface 26 and the arcuate portion 52of housing 12, within the formation zone 40. The formation zone 40extends circumferentially between an upstream end 54, which correspondsto the location at which the pulp 32 is initially applied to drum 14,and a downstream end 56 which corresponds to the location where the pulpmat 42 pulls away from portion 52 of housing 12. The low consistencypulp 32 is supplied to the pulp inlet 34 of housing 12, and then to theformation zone 40, from a conventional pump (not shown). The pumpestablishes a hydraulic pressure of the low consistency pulp 32 withinthe formation zone 40 which gradually forces the contaminated liquor outof the pulp mat 42, through holes 24 formed in the outer wall 16 of drum14, and into the individual ones of the filtration chambers 22 which arealigned with the formation zone 40. Each of the filtration chambers 22are sealed at one longitudinal end (not shown) of press 10 and are opento atmosphere at the opposite longitudinal end (not shown), therebypermitting the contaminated liquor entering filtration chambers 22 todrain out of press 10. Additionally, the difference between thehydraulic pressure of the pulp within the formation zone 40 and theatmospheric pressure within the filtration chambers 22 provides thenecessary force to dewater the pulp mat 42 as it passes throughformation zone 40. Accordingly, the consistency of the pulp within thepulp mat 42 is gradually increased as the pulp mat 42 travelscircumferentially through formation zone 40. As shown in FIG. 1, theformation zone 40 is divergent between the upstream end 54 and thecircumferentially spaced downstream end 56. The fact that the formationzone 40 is divergent, distinguishes formation zone 40 over the formationzones of conventional pressure-type pulp washers known to the inventor,which include convergent formation zones. Due to the gradual increase inconsistency of the pulp as the pulp mat 42 passes through the divergentformation zone 40, the space required to contain the pulp mat 42 isdecreased. Accordingly, the pulp mat 42 breaks away from the arcuateportion 52 of housing 12 at the downstream end 56 of the divergentformation zone 40 as shown in FIG. 1. The circumferential location ofthe downstream end 56 of zone 40 may vary with the production feed rateof the low consistency pulp 32 as subsequently discussed in greaterdetail.

After passing through the formation zone 40, the pulp mat 42 enters theextraction zone 44 which extends circumferentially between the formationzone 40 and the displacement zone 46, where a wash medium is applied tothe pulp mat 42 as subsequently discussed in greater detail. Theradially outer boundary of the extraction zone 44 is defined at anupstream end of extraction zone 44 by the first arcuate portion 52 ofhousing 12 and is defined throughout the remainder of extraction zone 44by a second portion 58 of housing 12 which is connected to the firstportion 52 of housing 12. Portions 52 and 58 of housing 12 blendsmoothly with one another and may comprise a unitary construction. Washpress 10 further includes a gas inlet 60 which communicates with theextraction zone 44. A gas 62, preferrably comprising air, is suppliedfrom a conventional blower (not shown) through the gas inlet 60 and intoextraction zone 44 so as to pressurize the portion of zone 44 whichexists between the pulp mat 42 and the portion 58 of housing 12 which isradially spaced from pulp mat 42. The extraction zone 44 is disposedcircumferentially on either side of the top center of housing 12 and thepressure of the air 62 entering zone 44 as well as the effects ofgravity combine to hold the pulp mat 42 against the drum 14 throughoutthe extraction zone 44. The pressurized air 62 creates a pressuredifferential across the mat 42 since the filtration chambers 22 areexposed to atmospheric pressure as discussed previously. The pressure ofair 62 gently forces contaminated liquor out of the pulp mat 42, throughholes 24 and into the individual ones of filtration chambers 22 whichare aligned with the extraction zone 44, for any given instant in time,with these chambers being denoted 22A-22E in FIG. 1. Removal ofcontaminated liquor from the pulp mat occurs gradually throughout theextraction zone 44. As shown in FIG. 1, filtration chambers 22A and 22Bare substantially filled with contaminated liquor which has been forcedout of the mat 42. In contrast, filtration chambers 22C, 22D, and 22Eare partially filled with contaminated liquor and each contains a gasspace, which reflects an amount of air which has passed through the pulpmat 42 as the pulp mat 42 travels clockwise through extraction zone 44.The pulp within pulp mat 42 reaches a terminal consistency which may beas high as about 15% o.d. prior to the pulp entering the displacementzone 46. Consequently, the contaminated liquor displaced into chambers22A-22E may be drained from drum 14 prior to these chambers rotatinginto the displacement zone 46, thereby providing effective separation ofthe subsequently described wash medium and the extracted, contaminatedliquor. The terminal consistency of the pulp as the pulp mat 42discharges from extraction zone 44 is achieved without disrupting thepulp mat 42 and accordingly, this consistency is achieved whilemaintaining the desired uniformity and integrity of pulp mat 42.

After discharging from the extraction zone 44, the pulp mat 42 entersthe displacement zone 46 where efficient displacement washing takesplace. The displacement zone 46 extends circumferentially between theextraction zone 44 and the compaction zone 48. Wash press 10 furtherincludes a baffle 64 which is pivotally attached to housing 12 by anupper pin 63 and a lower pin 66. Baffle 64 is disposed within housing 12and includes a circumferentially and longitudinally extending portion65, having a radially outer surface 67 and a radially inner surface 68,and a circumferentially and radially extending rib 69 attached toportion 65. The radially inner surface 68 of portion 65 comprises aradially inner surface of baffle 64 and forms a radially outer boundaryof the displacement zone 46. As shown in FIG. 1, baffle 64 convergesrelative to drum 14, between an upstream end 70 of baffle 64 and adownstream end 72 of baffle 64. A pneumatically actuated piston 74 isattached to the rib 69 of baffle 64 proximate the downstream end 72 ofbaffle 64, and is effective for pivoting baffle 64 between an operatingposition shown in solid in FIG. 1 and a retracted, fully divergent,pulp-plug-purging position indicated generally at 76 with dashed linesin FIG. 1. The fully divergent position 76 of baffle 64 may be achievedsince both the portion 65 and the rib 69 of baffle 64 include separateupper and lower portions which extend above and below, respectively, thelower pin 66. The radially inner surface 68 of baffle 64 is radiallyspaced from the outer surface 26 of the outer wall 16 of drum 14throughout the displacement zone 46. The area created by this spacing ofthe inner surface 68 of baffle 64 from drum 14 is at least partiallyflooded with a wash liquid 78 which is supplied to the displacement zone46 via a wash liquid inlet 80 which communicates with the displacementzone 46. In the illustrative embodiment shown in FIG. 1, inlet 80includes an outer portion 83 which is attached to housing 12, an innerportion 85 attached to baffle 64 and moveable therewith, and a flexibleportion 87 disposed between and attached to the outer portion 83 and theinner portion 85. Wash liquid 78 has a liquid level 79 which indicatesthe extent to which the space between surface 68 of baffle 64 and drum14 is flooded with the wash liquid 78 for typical production flow ratesof pulp 32 and wash liquid 78. However, the level of liquid 78 may varywith these production flow rates as subsequently discussed in greaterdetail. The beginning, or upstream end of the displacement zone 46corresponds to the location where the wash liquid 78 contacts the pulpmat 42, indicated by liquid level 79, and the downstream end ofdisplacement zone 46 corresponds to the location where baffle 64initially contacts the pulp mat 42. The wash liquid 78 may comprisesubstantially clean water or may alternatively comprise filtrate from asucceeding washer 10, when a plurality of washers 10 are used in aseries flow configuration. As yet another alternative, the wash liquid78 may comprise filtrate from a succeeding stage of the associated pulpplant. The combination of the static head of the wash liquid 78 and thepressure of the air 62 supplied to the extraction zone 44 combine toforce the wash liquid 78 through the pulp mat 42 so as to displacecontaminated liquor from the pulp mat 42. The use of piston 74 to loadbaffle 64 provides a controllable force which can be applied to the pulpmat 42 which permits the discharge consistency of the pulp within pulpmat 42 to be controlled independently of process variations. Thedischarge consistency of the pulp can be controlled in this manner tosuit the system requirement through a range of medium to highconsistency. The displaced, contaminated liquor flows through holes 24and into the filtration chambers 22 which are aligned with thedisplacement zone 46. The contaminated liquor then discharges out of theopen longitudinal end of press 10.

After passing through the displacement zone 46, the pulp mat 42 rotatesinto the compaction zone 48. The upstream end of compaction zone 48 isdefined by the location where baffle 64 initially contacts the pulp mat42 and the downstream end of compaction zone 48 is defined by thelocation where the pulp mat 42 discharges from the downstream end 72 ofbaffle 64. The radially inner surface 68 of baffle 64 is in contactingengagement with the pulp mat 42 throughout the compaction zone 48.Accordingly, a mechanical force is applied to the pulp mat 42 whichdisplaces an additionally quantity of contaminated liquor from the pulpmat 42, with the magnitude of the applied mechanical force increasing ina clockwise direction due to the convergent relationship between baffle64 and drum 14. The convergence rate of the baffle 64 relative to drum14 is gradual so that the portion of the wash liquid 78 absorbed by thepulp mat 42 in a stratified manner is uniformly drawn through the pulpmat 42 maintaining the displacement process throughout the compactionzone 48. If a pulp plug forms within the compaction zone 48, baffle 64can be retracted to the fully divergent position indicated at 76. Sincethe pulp mat 42 is removed from drum 14 after it discharges from thedownstream end 72 of baffle 64, as subsequently discussed in furtherdetail, there are no additional elements of press 10 which can impedethe purging action of any pulp plug which is formed. Furthermore, due tothe positioning of baffle 64 the purging of any pulp plug which isformed is facilitated by the action of gravity and the static pressurehead of the wash liquor 78.

As discussed previously, the extraction zone 44 extendscircumferentially between the formation zone 40 and the displacementzone 46. Accordingly, the arc length of the extraction zone 44 isdetermined by the circumferential spacing between the downstream end ofthe formation zone 40 and the upstream end of the displacement zone 46.It may be appreciated that as the feed rate of the low consistency pulp32 supplied to the pulp inlet 34 of housing 12 is increased, and thecorresponding feed rate of the wash liquor 78 is increasedproportionally, that the arc length of both the formation zone 40 andthe displacement zone 46 will necessarily increase. Since the pulp inlet34 and the downstream end 72 of baffle 64 are at fixed locations inpress 10, the arc length of the extraction zone 44 is foreshortened byan amount corresponding to the combined increase in the arc lengths ofthe formation zone 40 and the displacement zone 46. This phenomenon cancontinue until the formation zone 40 meets the displacement 46, suchthat the extraction zone 44 is eliminated. In this instance, theinterface between the formation zone 40 and the displacement zone 46occurs within an upper portion of housing 12, where a tangent to theouter surface 26 of the outer wall 16 of drum 14 is substantiallyhorizontal. In this mode of operation, the wash liquid 78 has a liquidlevel 81 indicated in dashed lines, which extends between the pulp mat42 and the inner surface 68 of baffle 64. Furthermore, the pulp mat 42breaks away from housing 12 at approximately this location, which isindicated generally at 57 shown in dashed lines. Since the interfacebetween the formation zone 40 and the displacement zone 46 occurs withinan upper portion of housing 12 in this instance, the effects of gravityassist in maintaining the integrity of the pulp mat 42 at the locationof initial contact with the wash liquid 78, as indicated by liquid level81. Furthermore, the circumferential location of the wash liquid inlet80 is below the liquid level 81 when the arc length of extraction zone44 is sufficiently foreshortened, or is nonexistent, due to the increasein the arc lengths of the formation zone 40 and the displacement zone46. As the upstream end of the displacement zone 46 movescounterclockwise the liquid level of wash liquid 78 also movescounterclockwise which is indicated by the difference in locations ofliquid levels 79 and 81 shown in FIG. 1. As this occurs, the radialclearance between the inner surface 68 of baffle 64 and the pulp mat 42increases. The combination of this increasing clearance between surface68 and pulp mat 42 and the location of wash liquid inlet 80 relative toliquid level 81, allows any turbulence of the wash liquid 78 to dampenout, thereby eliminating the potential of the wash liquid 78 to disruptthe pulp mat 42. Additionally, this mode of operation eliminates airentrainment which may be otherwise introduced into the pulp mat 42within the extraction zone 44, thereby maintaining the uniformity of thepulp mat 42. The foregoing features of press 10 enable a wide range ofproduction rates without compromising operability or efficiency.

After the pulp mat has discharged baffle 64, the pulp mat ismechanically doctored, or removed from drum 14 by a doctor blade 91which is attached to housing 12 and disposed within the face-cleaningand take-off zone 50. The tip of the doctor blade 91 is spaced in closeproximity to the outer surface 26 of the outer wall 16 of drum 14. Blade80 is effective for breaking the pulp mat 42 into small pieces,indicated generally at 82 which fall into a pressurized repulper, oroutlet chute 84. A helical screw 86 is rotatably mounted within theoutlet chute 84 and is used to discharge the pulp from chute 84. Adilution liquid 88, which may comprise water or a variety of washfiltrates, is supplied to chute 84 via dilution liquid inlet 90.Dilution liquid 88 is used to achieve the desired pulp consistency whichis consistent with requirements of the downstream process. The pulpdischarges out of chute 84 through a chute outlet port 92, conduit 94and valve 96 which is disposed within conduit 94. The pulp may then besupplied to a downstream process as indicated by flow arrow 98. Thelevel 89 of the diluted, washed pulp stock within chute 84 may becontrolled to a desired height by opening valve 96 to a predeterminedposition.

Although the doctor blade 91 removes a substantial portion of the pulpmat 42 from drum 14, small particles of pulp, indicated generally at 100pass through the small clearance formed between the tip of the doctorblade 91 and drum 14 and then drop through a discharge port 102 of acrumb sluice box 104. A spray manifold 106, having at least one spraynozzle 108 is disposed within the crumb sluice box 104. A wash liquidflowing through the hollow interior of the manifold 106 is directed ontothe outer surface 26 of the outer wall 16 of drum 14 by nozzle 108 so asto clean the surface 26 within the face-cleaning and take-off zone 50 ofpress 10. The face-cleaning and take-off zone 50 begins as the pulp mat42 discharges from the downstream end 72 of baffle 64 and ends at theupstream end of the formation zone 40. Contaminated filtrate, or liquor,contained within the individual filtration chambers 22 which are alignedwith the face-cleaning and take-off zone 50 for any given instant intime, with these chambers being denoted as 22F and 22G in FIG. 1, willbegin to flow outward through the holes 24 which communicate withchambers 22F and 22G and extend through the outer wall 16 of drum 14,under the action of gravity and centrifugal forces. The contaminatedfiltrate is substantially prevented from contacting the washed pulp 82by the doctor blade 91 and is directed into the crumb sluice box 104 andout of the press 10 through the discharge port 102. Due to therotational speed of the drum 14, and the relatively smallcircumferential spacing between the downstream end 72 of the baffle 64and the tip of the doctor blade 91, re-wetting of the washed pulp 82 bythe contaminated filtrate within chambers 22F is insignificant.

FIG. 2 illustrates a single roll displacement wash press 110 accordingto a second embodiment of the present invention. Wash press 110 isidentical to wash press 10, with the following exceptions. Wash press110 includes a seal 111 which has one end attached to outlet chute 84and an opposite end disposed in sealing engagement with the doctor blade91. The crumb sluice box 104 of wash press 10 is replaced by a member113 which extends between the outlet chute 84 and the pulp slurry inletbox 36. Seal 111 and member 113 combine with the doctor blade 91 to forma chamber 115 radially outward of drum 14 within the face-cleaning andtake-off zone 50. Spray manifold 106 of wash press 10 is eliminated anddilution inlet 90 of wash press 10, which communicates with outlet chute84, is replaced by a dilution liquid inlet 117 which is attached tomember 113 and communicates with the face-cleaning and take-off zone 50.The seal 111 which extends between doctor blade 91 and the outlet chute84 forces the dilution liquid 88, supplied to inlet 117, through theclearance between the tip of the doctor blade 91 and the outer surface26 of the outer wall 16 of drum 14, with this clearance being increasedrelative to that of wash press 10. The injection of liquid 88 betweenthe tip of the doctor blade 91 and drum 14 facilitates removal of themat 42 from drum 14. The dilution liquid 88 fills the outlet chute 84and the chamber created between housing 12 and the radially outersurface 67 of the circumferentially and longitudinally extending portion65 of baffle 64 over an arc length corresponding to the compaction zone48 and at least a portion of the displacement zone 46, as indicated bylevel 121 of the liquid 88. The level 121 of liquid 88 is controlled tothe desired level by control valve 96. Due to the hydraulic pressure ofthe dilution liquid 88 exerted on the radially outer surface 67 ofportion 65 of baffle 64, reduced force is required to maintain baffle 64in the operating position. Accordingly, piston 74 may have a reducedsize, as compared to that required for use with wash press 10.Additionally, since outlet chute 84 is flooded with the dilution liquid88 air is not entrained in the pulp discharging from outlet chute 84,which enhances the pulp for use in downstream processes. It is notedthat wash press 110 may also operate in the mode where the extractionzone 44 is eliminated, due to relatively high pulp and wash liquidproduction feed rates, as discussed previously with respect to washpress 10.

In operation, both wash press 10 and wash press 110 receive a flow ofpulp 32 having a consistency ranging from about 2% o.d. to about 6% o.d.In each of the wash presses 10 and 110, a pulp mat 42 is formed on theouter surface of drum 14, with the pulp mat 42 being exposed insuccession to the formation zone 40, the extraction zone 44, thedisplacement zone 46 and the compaction zone 48, as drum 14 rotatesabout centerline axis 30. Presses 10 and 110 are effective fordisplacing the undesirable contaminates from the pulp mat 42, andincreasing the consistency of the pulp within mat 42, as mat 42 travelsthrough each of the foregoing zones. The use of the divergent formationzone 40, followed immediately by the extraction zone 44 where thecontaminated liquor is gently forced out of the pulp mat 42 by theaction of the pressurized gas 62 within zone 44 avoids the problem ofpulp mat disruption experienced by prior wash presses utilizingconvergent formation zones followed immediately by wash zones. Thehomogeneity and terminal consistency of the pulp within the pulp mat 42discharging from the extraction zone 44 provides ideal conditions forefficient displacement washing within the displacement zone 46. In theinstances where the extraction zone 44 is eliminated from wash presses10 and 110 due to relatively high pulp and wash liquid feed rates,disruption of the pulp mat 42 is minimized since the interface betweenthe formation zone 40 and the displacement zone 46 occurs within anupper portion of housing 12 during operation in this mode. Furthermore,operation in this mode eliminates air entrainment which may otherwise beintroduced when the pulp mat 42 passes through the extraction zone 44.Wash presses 10 and 110 provide a wide range of pulp dischargeconsistencies and provide effective means for purging any pulp fiberplugs which may accumulate since baffle 64 may be retracted to thepulp-plug-purging position 76 and no other converging areas exist withinthe presses. Wash press 1 10 provides the additional advantages ofreduced piston force required to load baffle 64 relative to thatrequired of wash press 10, avoidance of induced air entrainment in thepulp within the outlet chute 84, and elimination of the crumb sluice box104.

While the foregoing description has set forth the preferred embodimentsof the present invention in particular detail, it must be understoodthat numerous modifications, substitutions and changes can be undertakenwithout departing from the true spirit and scope of the presentinvention as defined by the ensuing claims. The invention is thereforenot limited to specific preferred embodiments as described, but is onlylimited as defined by the following claims.

What is claimed is:
 1. A method for increasing a consistency of pulpusing a single roll displacement wash press having a housing and a drumrotatably mounted within the housing, the press further including incircumferential succession, a formation zone, an extraction zone, and adisplacement zone, said method comprising the steps of:supplying a pulphaving a consistency ranging from about 2% o.d. to about 6% o.d. to theformation zone of the single roll displacement wash press; forming a matof the pulp on a generally cylindrical outer surface of the drum withinthe formation zone of the press, the formation zone comprising anupstream end and a downstream end, the formation zone being divergentbetween the upstream end and the downstream end; supplying a gas to theextraction zone so as to pressurize the extraction zone and gently forcecontaminated liquor out of the pulp mat so as to increase theconsistency of the pulp within the extraction zone; washing the pulp matwithin the displacement zone of the press; removing the pulp mat fromthe drum.
 2. The method of claim 1 wherein the supplying step furthercomprises increasing the consistency of the pulp within said extractionzone to about 15% o.d.
 3. The method of claim 1 wherein the washing stepfurther comprises flooding said displacement zone with a wash liquidwhich is effective for displacing contaminated liquor from said pulpmat.
 4. The method of claim 1 wherein the removing step furthercomprises removing at least a substantial portion of the pulp mat fromsaid drum with a doctor means and dropping the pulp mat into an outletchute of said housing after being removed from said drum by said doctormeans.
 5. The method of claim 4 further comprising the step ofextracting the pulp mat from said outlet chute of said housing with ahelical screw rotatably mounted within said outlet chute.
 6. The methodof claim 4 further comprising the step of forcing a dilution liquidbetween said doctor means and said drum so as to facilitate removal ofthe pulp mat from said drum and for flooding said outlet chute so as tominimize air entrainment within the pulp.
 7. A method for increasing aconsistency of pulp using a single roll displacement wash press having ahousing and a drum rotatably mounted within the housing, the pressfurther including in circumferential succession, a formation zone, anextraction zone, and a displacement zone, said method comprising thesteps of:supplying a pulp having a consistency ranging from about 2%o.d. to about 6% o.d. to the formation zone of the single rolldisplacement wash press; forming a mat of the pulp on a generallycylindrical outer surface of the drum within the formation zone of thepress, the formation zone comprising an upstream end and a downstreamend, the formation zone being divergent between the upstream end and thedownstream end; extracting contaminated liquor out of the pulp mat inthe extraction zone so as to increase the consistency of the pulp withinthe extraction zone; washing the pulp mat within the displacement zoneof the press; removing the pulp mat from the drum.
 8. The method ofclaim 7 wherein the extracting step further comprises supplying a gas tothe extraction zone to gently force contaminated liquor out of the pulpmat.
 9. The method of claim 7 wherein the supplying step furthercomprises increasing the consistency of the pulp within said extractionzone to about 15% o.d.
 10. The method of claim 7 wherein the washingstep further comprises flooding said displacement zone with a washliquid which is effective for displacing contaminated liquor from saidpulp mat.
 11. The method of claim 7 wherein the removing step furthercomprises removing at least a substantial portion of the pulp mat fromsaid drum with a doctor means and dropping the pulp mat into an outletchute of said housing after being removed from said drum by said doctormeans.
 12. The method of claim 11 further comprising the step ofextracting the pulp mat from said outlet chute of said housing with ahelical screw rotatably mounted within said outlet chute.
 13. The methodof claim 11 further comprising the step of forcing a dilution liquidbetween said doctor means and said drum so as to facilitate removal ofthe pulp mat from said drum and for flooding said outlet chute so as tominimize air entrainment within the pulp.